Electronic equipment, particularly sensitive electronic equipment such as computers and communications equipment are all susceptible to malfunction as a result of electromagnetic wave interference. In addition to being sensitive to foreign electromagnetic wave interference, many of these devices generate electromagnetic wave interference. Various methods have been used to impart electromagnetic wave interference shielding to electronic equipment enclosures. Typically, shielding of electronic equipment enclosures is achieved by one or more of three major techniques, i.e., utilizing metal enclosures which, are inherently conductive; utilizing plastic molded enclosures having an electroconductive surface, for example, provided by a conductive film, plating or conductive paint; and molding a conductive plastic enclosure from a polymer containing electroconductive materials.
Attempts have been made to prepare conductive plastics by incorporating certain conductive fillers into engineering thermoplastics. Specifically, these fillers include conductive powders, flakes and fibers. More recently, attempts have been made to find synergistic combinations of conductive fillers so as to provide extrudeable and/or moldable compounds having consistent shielding at lower loadings which maintain properties in the finished molded article. Such combinations include metal fibers and carbon fibers, metal flake and/or carbon fibers in combination with carbon black powder, metal flake and metal or metal-coated fiber, and metal flake and/or metal and/or metal-coated fiber with conductive carbon powder.
Although these combinations have high overall electromagnetic wave interference shielding effectiveness, they suffer from such problems as deteriorating the physical and aesthetic properties of the polymer. Further, the levels of conductive filler required to meet electromagnetic wave interference shielding effectiveness often result in a polymer with such high viscosity it can not practically be molded into electronic equipment enclosures, especially some of the recent thin wall enclosures, such as those found in hand held phones or computers. Another serious problem is the breakage of electroconductive fibers due to shearing and kneading of a polymer and electroconductive fibers to prepare pellets for molding and resulting in lowering the shielding effectiveness in proportion to the degree of breakage. Thus, it is necessary to increase the amount of the electroconductive fibers in the polymer in anticipation of the breakage of the electroconductive fibers, but this will bring about such secondary problems as lowering of the productivity and an increase on the weight of the molded article.